diff --git a/include/villas/log.h b/include/villas/log.h
index e5699203e..5234f71bd 100644
--- a/include/villas/log.h
+++ b/include/villas/log.h
@@ -98,6 +98,7 @@ struct log {
 	int level;
 	long facilities;	/**< Debug facilities used by the debug() macro. */
 	const char *path;	/**< Path of the log file. */
+	char *prefix;		/**< Prefix each line with this string. */
 	FILE *file;		/**< Send all log output to this file / stdout / stderr. */
 };
 
diff --git a/lib/Makefile.villas.inc b/lib/Makefile.villas.inc
index 49090e7fb..9bc9afffb 100644
--- a/lib/Makefile.villas.inc
+++ b/lib/Makefile.villas.inc
@@ -33,7 +33,7 @@ LIB_SRCS += $(addprefix lib/kernel/, kernel.c rt.c) \
                utils.c super_node.c hist.c timing.c pool.c list.c queue.c \
                queue_signalled.c memory.c advio.c plugin.c node_type.c stats.c \
                mapping.c io.c shmem.c config_helper.c crypt.c compat.c \
-               log_table.c log_helper.c io_format.c periodic_task.c \
+               log_table.c log_helper.c io_format.c task.c \
             )
 
 LIB_LDFLAGS = -shared
diff --git a/lib/io.c b/lib/io.c
index 0bd7d85ea..6abf70bff 100644
--- a/lib/io.c
+++ b/lib/io.c
@@ -75,9 +75,9 @@ int io_stream_open(struct io *io, const char *uri)
 			if (io->advio.output == NULL)
 				return -1;
 
-			io->advio.input  = afopen(uri, "r");
+			io->advio.input = afopen(uri, "a+");
 			if (io->advio.input == NULL)
-				return -1;
+				return -2;
 		}
 	}
 	else {
diff --git a/lib/io_format.c b/lib/io_format.c
index 5c9dc170c..bbf3d1bd8 100644
--- a/lib/io_format.c
+++ b/lib/io_format.c
@@ -27,11 +27,16 @@
 
 int io_format_sscan(struct io_format *fmt, char *buf, size_t len, size_t *rbytes, struct sample *smps[], unsigned cnt, int *flags)
 {
+	if (!flags)
+		flags = &fmt->flags;
+	
 	return fmt->sscan ? fmt->sscan(buf, len, rbytes, smps, cnt, flags) : -1;
 }
 
 int io_format_sprint(struct io_format *fmt, char *buf, size_t len, size_t *wbytes, struct sample *smps[], unsigned cnt, int flags)
 {
+	flags |= fmt->flags;
+
 	return fmt->sprint ? fmt->sprint(buf, len, wbytes, smps, cnt, flags) : -1;
 }